European Patent Application No. 136926 A provides a process for distilling air in a conventional double column which comprises a distillation column operating at a relatively low pressure, a second distillation column operating at a relatively high pressure and a condenser-reboiler which provides condensate as reflux to the relatively high pressure column and reboiled liquid gas to the lower pressure column. Liquid oxygen is taken from one of the columns and is passed to the top of an auxiliary column operating substantially at the pressure of the lower pressure column, a gas less rich in oxygen than the liquid oxygen is taken from the lower pressure column and is passed to the bottom of the auxillary column, and the liquid collected at the bottom of the auxiliary column is passed as reflux into the low pressure column substantially at the level from which the said gas is withdrawn. One of the advantages offered by this process is that when a surplus of oxygen is produced, that is when the rate of production of oxygen is greater than the demand for it, the excess liquid oxygen can, if effect, be used to increase the reflux to the lower pressure column thereby enabling an increase to be made in the amount of argon-enriched fluid that is withdrawn from the lower pressure column for subsequent processing, typically in a further distillation column, to produce a crude argon product.
The present invention provides an alternative method and apparatus which enhances argon production by passing the aforementioned reflux to the argon column rather than to the lower pressure column.